In recent years, non-contact information media, such as IC cards (smart cards), have been put into practical use. Such media supply power via radio waves by utilizing a mutual induction effect of coils and transmit/receive data.
As examples of non-contact information media, non-contact IC cards are broadly categorized into close-coupled IC cards, proximity IC cards, vicinity IC cards, and microwave IC cards based on the communication distances between the IC cards and readers/writers for transmitting/receiving radio waves to/from the IC cards. More particularly, the close-coupled IC cards have a communication distance of 0 through 2 mm, the proximity IC cards have a communication distance of 0 through 10 cm, the vicinity IC cards have a communication distance of 0 through 70 cm, and the microwave IC cards have a communication distance of 0 through 10 m. The international standardization of each type of the IC cards has been almost prepared. Technical difficulty increases with an increase in the communication distance. The close-coupled IC cards do not have any significant difference in their characteristics from already widely used contact IC cards. Therefore, it cannot be said that good use is made of the merits of non-contact IC cards. Accordingly, it is proximity non-contact IC cards that are expected to be widely used immediately after the contact IC cards.
The proximity IC cards that can be used at a distance of 0 through 10 cm from readers/writers may be quite extensively applicable. For example, if the proximity IC cards are used for commuter passes, information exchanges between the proximity IC cards and readers/writers in a non-contact manner allow the opening/closing of ticket gates of stations to be controlled without the need to take the IC cards out of commuter-pass holders at the ticket gates.
In order that the IC cards can extensively be applied, it is significant to make the IC cards smaller and lighter. It is expected that the more extensively the IC cards are applicable, the less care will be taken in handling the IC cards. Therefore, in view of less susceptibility to malfunction from rough handling, non-contact information media, such as IC cards, have normally been mounted with semiconductor integrated circuits each including a complicated circuit in a small area.
The following performance is demanded for semiconductor integrated circuits for non-contact IC cards. Supply voltages required to operate internal digital circuits (logic circuits) and nonvolatile memories with stability must be generated even at a distance of approximately 0 through 10 cm from the readers/writers to the non-contact IC cards, thereby obtaining stable demodulated signals. More particularly, when the distance between a reader/writer and a non-contact IC card is a short distance of 0 cm, i.e., the non-contact IC card is very close to the reader/writer, the integrated circuit mounted in the non-contact IC card will be broken unless the supply voltage supplied to the internal digital circuit and the nonvolatile memory is below specification (for example, 5.5 volts or less for 5-volt tolerant integrated circuits). Furthermore, in order to obtain a stable demodulated signal, the input voltage to the demodulation circuit need be prevented from reaching an overvoltage.